In spite of the best that modern ingenuity can do to complicate cryptography, the art of deciphering cryptograms has well-nigh kept pace with it. Indeed, it is hardly too much to say that a code based upon any regular mathematical principle may be solved by ingenuity and pa tience. If the language of the document is known in advance, the relative frequency with which the letters of the alphabet normally occur in that language forms an important initial clue. Thus c is the letter of most frequent occurrence not only in our own language. but in French and German as well. In English the next in order of frequency are t, o, a, r, s, h, d, 1, c, w, u, m, etc. Single letters must be either a, i, or o. Words of two letters most likely to occur are of, to, in, it, is, be, he, by, or, etc. Double letters are most apt to be ee, (Jo. ff, 11, or ss. If there is doubt whether the cipher is in Latin, English, French, or German, the lack of double letters at the end of words suggests that it is Latin; if but few words end with double letters, it is probably French: if they are very numerous. it is German. A highly' inflected language, like Latin, or even the Romance languages, with their com plicated conjugations, must be easier to solve than English is, on account of the regular recurrence of the same combinations of letters in the inflec tional terminations. Those who make a science
of interpreting cipher documents receive no small assistance from a knowledge of the frequency with which certain symmetrical combinations of letters occur in the vocabulary of a language. Thus, the combination which may be represented for convenience by the formula abab is compara tively rare in English: one may cite papa, dodo; in French, tete, hebe. The form abeba is found in lerel: Fr., rover. In German, the formula ablat gives only Anna, Ebbe, Egge, Esse, Otto: in French, the formula abedahe gives only two words. cherehe and (pelvic.
Besides the' writers on cryptography already mentioned, readers interested in the subject may he referred to John Baptist Porta, Dr Furtivis Litcrarum Tot is (15631 ; Blaise de Vigenere, Traite des ehiffras (1587 ) ; Thicknesse, A Treat ise on the Art of Deciphering and of Writing in Cipher (1772) ; and among more modern writers, J. L. Kluber, Kryptographi• (Tiibingen. 1S09) ; Romani. La cryptographic Vroilee (1875) ; and Fleissner, Hamlburh der Kryptographik (Vienna, 1881 ) .